Field of the Invention
The present invention relates to a turbocharger for an internal combustion engine. More particularly, it relates to a construction of combined metal-ceramic turbine wheel for the turbocharger.
The turbine wheel of a turbocharger for an internal combustion engine operates under severe thermal conditions caused by contact with the extremely high temperatures of the exhaust gas from the internal combustion engine. Accordingly, the turbine wheel is usually made of a heat resistant metal material, such as Inconel. The heat resistance of such a material is, however, insufficient to fully resist these high temperatures.
To overcome this heat resistance insufficiency, it has been proposed to make the turbine wheel of a high heat resistance ceramic material. In this case, however, it is difficult to cast the turbine wheel as a one-piece unit including a shaft portion, since ceramic is fragile when subjected to a shearing force, and such a force is exerted mainly at a portion where the turbine wheel is supported in a housing. The one-piece construction is also disadvantageous from the viewpoint of cost.
In view of the above, it has been proposed to construct a turbine wheel having a turbine wheel portion made of a ceramic material and a shaft made of a metal material, as disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 54-42520. In this prior art, the turbine wheel made from a ceramic material is provided with a blade portion and a support portion extending axially from the blade portion. The support portion of the turbine wheel is thermally fitted inside a hollow shaft made from a metal material. This prior art, however, has a drawback in that the turbine wheel may become loose in the shaft or be damaged due to the difference in the thermal expansion coefficient between the hollow shaft made from metal and the turbine wheel made from ceramic. Namely, the highest temperature of the exhaust gas is usually transmitted to the location at which the turbine wheel is fitted to the shaft. In this case, since the thermal expansion of the metal shaft becomes greater than the thermal expansion of the ceramic turbine wheel, a linear thrust force is generated by the expansion of the hollow shaft in the axial direction, forcing the turbine wheel away from the shaft toward the exhaust gas outlet. This linear thrust force generates a shearing force at the location where the turbine wheel is inserted in the hollow shaft, causing the wheel to be loosened and displaced or damaged by uneven contact with the hollow shaft.